The present invention relates to organic electroluminescent devices. More particularly, it relates to using new cathode materials.
Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface at the HTL. Tang et al. (Applied Physics Letters, 51, 913 [1987], Journal of Applied Physics, 65, 3610 [1989], and commonly assigned U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed.
The cathode plays an important role in the overall performance of OLED devices. It injects electrons into the ETL during operation. However, there is often an adhesion problem between the metal cathode and the organic ETL in OLEDs. This rough interface between cathode and organic layer due to poor adhesion results in poor device performance, such as higher driving voltage, lower luminescent efficiency, and shorter operational lifetime.
The addressed adhesion problem between the metal cathode and the organic layer has been discussed in Tang and VanSlyke (U.S. Pat. No. 4,885,211). It was found that Mg does not adhere well to organic surface during thermal evaporation, which results in a high sheet resistance, rough film morphology, and poor device performance of Mg cathode devices. Tang and VanSlyke therefore disclosed magnesium alloy electrodes such as Mg:Ag (U.S. Pat. No. 4,885,211) and an Al:Mg (U.S. Pat. No. 5,059,862). The alloy cathodes have improved adhesion to ETL and are formed by co-evaporation of Mg with Ag or Al. With such Mg alloy cathodes, OLEDs can achieve better performance than devices having pure Mg as a cathode. However, fabrication of a Mg alloy cathode requires two thermal evaporation sources that must be precisely controlled to ensure the proper alloy ratio. The dual source evaporation is necessarily more complicated than single source evaporation. As for Al:Mg cathode, thermal evaporation of Al with suitable evaporation rate is difficult to achieve.
There have been numerous disclosures of using alkali metal compounds (U.S. Pat. No. 5,739,635), alkaline earth metal compounds (U.S. Pat. No. 5,457,565), and other materials (U.S. Pat. No. 6,013,384) as an electron-injecting layer (EIL) for use with Al cathodes. Lithium compounds in particular have been widely disclosed as a useful EIL material for Al cathodes. However, substantially pure Mg has several desirable features including a low-work function (xcx9c3.7 eV) and excellent thermal evaporation properties. The use of substantially pure Mg as a cathode in high efficiency OLEDs has not been demonstrated.
It is an object of the present invention to provide an OLED with good adhesion between a substantially pure magnesium cathode and organic layer.
It is another object of the present invention to provide OLED with an evaporated magnesium cathode.
It is another object of the present invention to improve the efficiency, driving voltage, and operational stability of OLED.
These objects are achieved in an organic electroluminescent device comprising:
a) an anode and a cathode;
b) an electroluminescent medium disposed between the anode and the cathode;
c) an adhesion-promoting layer in contact with the cathode and the electroluminescent medium;
d) the adhesion-promoting layer has a thickness of between 0.01 to 3.0 nm and comprises at least one metal or metal compound selected from group 1 through group 15 of the Periodic Table of Elements such that the metal has an atomic number of at least 19; and
e) the cathode is substantially pure magnesium.
An advantage of the present invention is that a substantially pure Mg cathode can be made useful in high efficiency OLEDs. It has been found quite unexpectedly that the adhesion-promoting layer, in the thickness range from 0.01 nm to 3.0 nm, can significantly improve the adhesion of Mg on the organic EL medium.
Another advantage of the present invention is that the EL efficiency, driving voltage, and operational stability of the OLEDs can also be improved by disposing the adhesion-promoting layer between ETL and the substantially pure Mg cathode.